a bullet going with speed 350m/s enters a concrete wall and penerate a distance of 0.5cm before coming to rest.find decelaration.

You have initial velocity, final velocity, and displacement. Therefore, you could use:
\[a=\frac{v_1-v_0}{t}\]and\[s=v_0t+\frac{1}{2}at^2\], which should allow you to find t, which would further allow you to find a!